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Treetops

Treetops

Look at a tree. Winter is best for it. Without that distracting screen of leaves, the tree's shape is clear against the sky—what Kim Steiner likes to call its architecture.

Steiner steps past me to the window, picks out a pin oak on the lawn, with its oval-shaped crown. A sequoia, conical, with its whirl of small branches. He's a matter-of-fact man, Steiner. A professor of forest biology at Penn State, he teaches dendrology: no surprise he knows his trees. "Different species can be recognized easily by how their branches go together," he says.

Not by their bark. Not by their leaves.

"It's the architecture of this branch-and-trunk scaffold that is really distinctive at a distance."

He draws back from the window, regains his seat beside the gleaming surface of a large cherry-wood desk.

"You tend to think of branching as a random process," he continues, "irregular, stochastic. And there are random processes at work—storm breakage, shading by another tree, insect damage. Yet still there's this sort of architectural pattern the species adheres to.

"That's what fascinates me . . . this pattern."

It's also a practical matter. Air pollution studies, Steiner notes, are mostly done on greenhoused seedlings, yet "the seedling is not the forest." Specifically, a seedling's forks and branches—its architecture—look quite unlike those of trees.

Thus Steiner's part of a current $500,000 EPA study of ozone, co-directed by Penn State plant pathologist John Skelly, addresses what Steiner calls this "scaling" problem. As his former graduate student, Durland Shumway, explained in his 1991 dissertation, "As a tree grows from seedling to maturity, it must 'solve' the type of problem that architectural engineers encounter when increasing the size (or scale) of a model."

"If you could characterize the architecture of a seedling well enough," muses Steiner, "perhaps it might be possible to see the future tree in the way the seedling is growing."

To Leonardo da Vinci, a tree was a river on end.

"All the branches of a tree at every stage of its height when put together are equal in thickness to the trunk," he wrote in his notebook in 1510. "All the branches of a water at every stage of its course, if they are of equal rapidity, are equal to the body of the main stream."

"Da Vinci was interested in how to draw things," says Steiner, "but also in how things worked. He was making an analogy."

What does a tree do? It sucks water out of the earth and squirts it into the air. When a leaf opens its pores, its stomates, in order to take in the carbon dioxide it needs for photosynthesis, water vapor escapes; the water tension in the leaf rises. Pulled in to fill that vacuum in the leaf tissue, fresh water flows in through the xylem, the woody tissue, of the twig, to the twig from the branch, to the branch from the trunk, to the trunk from the roots, to the roots from the soil.

But a tree is not a river. The water flows inside.

In 1964, a quartet of Japanese scientists expanded on DaVinci in the Japanese Journal of Ecology. A tree, they argued, is a pipe or, more precisely, "an assemblage of 'unit pipes,'" according to Steiner's graduate student Brian Joyce, who quoted both DaVinci and the Japanese paper in his master's thesis. "As the tree develops, new unit pipes are added to the old pipe assemblage," Joyce wrote, explaining the Japanese theory. "The stem cross-sectional area is proportional to the area of foliage it supports."

"We discovered it's not the whole story," Steiner says. "Piping water is not all a tree does. It also has to support itself."

"Think of some fundamental problems," says Steiner.

"The strength of a branch at any given point is a function of the cross-sectional area of the branch." In the equation that describes it, Steiner explains, a branch's strength varies in direct proportion to the square of its diameter. "The weight of a branch, however, is a cubic function, a function of the diameter of the branch cubed. So, assuming the overall shape remains unchanged, the weight accumulates faster than the strength."

Were it merely a pipe, a woody fountain, this tree would come crashing down.

"A tree is not a pipe," says Steiner. "It's solid, with hollow places in it that conduct water.

"There are ways for it to compromise on its competing functions, its water-conducting function and its support function.

"The thing Durland found out—"

He gets out a lined pad and writes the equation for the Japanese pipe model, Pr02 = Pr12 + Pr22 with pi canceling out and r standing for radius, and the "radius exponent" fixed at 2. Then he writes the equation for da Vinci's river, Pr0 = Pr1 + Pr2, its radius exponent 1.

"It's neither."

Shumway, for his Ph.D. research, measured the length of each twig in the crowns of four black walnut trees, with a steel measuring tape, to the nearest 0.30 centimeter. He sliced a thin disc from each twig, measured each diameter with a dial caliper, twice. According to his calculations, in the crowns of black walnut trees the radius exponent is 1.8.

"That was really, really encouraging," says Steiner. "You can't call it conservation of cross-sectional area, because there's some diminishment. The twigs become more slender on the distal side of the bifurcation—they don't add up to the same amount of wood. Yet they're transporting the same amount of water.

"Durland and I understood we weren't going far enough with these simple morphological experiments, so we turned our attention to the water transport function itself.

"A tree is not a simplistic thing," he adds. "There are things going on inside the wood that we had to understand."

A chilly September morning, we head to the woods. Steiner and his current student, Joyce, meet me in a Jeep Cherokee (of a color Steiner calls "black cherry") and we drive north along I-80.

"They start to shut down as soon as they get pretty," Joyce says, with a nod at the reddening hillsides. "There's not much to measure."

Steiner turns into the Moshannon State Forest, passes the Bureau of Forestry district office (where Skelly has been monitoring atmospheric ozone for the past 10 years), hangs a left into an open field planted with a regiment of ash, bumps past a chewed-up plot from which whole saplings have been exhumed (the roots blasted out with a fire hose) to be taken back to the lab, and parks. Across the field, a workman Joyce identifies as Charlie, a renowned beaver trapper and eater of beaver tail, putters beside a high white wooden barn, preparing it for the weekend's firefighters' picnic.

Steiner gets out of the Jeep. "Tell her how a tree is put together," he tells Joyce.

For his master's thesis, Joyce had dissected one: cut trunk, branch, and twig into 3-inch lengths and pumped water through each length. "It was a youthful white ash," Steiner had told me earlier, "18-20 feet tall.

"This shoot," he wags the tip, a twig much longer than those along the sides of the branch, "is programmed to have a more efficient water transport system. This part of the tree is hydraulically favored."

In Joyce's thesis tree, the trunk always had a higher "leaf-specific conductivity"—a measure of the wood's ability to conduct water in proportion to the number leaves it was feeding—than any branch; a branch's LSC was always higher than any of its twigs.

And yet, over the length of the trunk, the LSC rose then fell again. "The branch had the same up and then down pattern over its length, almost as if it were a small-scale model of the whole tree," Steiner had explained.

Except at the treetop.

The tip's LSC was significantly higher than that of any other twig.

"What this means is that when the tree is experiencing severe water stress, the laterals," Joyce fingers the side-twigs of the field-grown ash, "will cavitate first—that means there's a breakage of the water colum—quot;

"In the '88 drought," adds Steiner, "you could go into this plantation and every leaf was dead except that top shoot. I wish I'd taken more pictures. If I'd known then that our hydraulic architecture research would actually explain what we were seein—quot;

LSC also explains why young trees grow first up and then out.

"The twig with the higher LSC is going to be favored," Steiner explains, as we regain the warmth of the car. "It will get more nutrients, it can keep its stomates open longer, it can absorb more carbon dioxide and photosynthesize more, all because it has a better water supply.

"And because this twig will produce more photosynthate, it will have larger buds. They will produce a larger shoot with more leaves. The more leaves, the more xylem—since the hormone that causes xylem to develop is made in the leaves—the more xylem, the more water and nutrients the leaves can ge—

"You get this positive feedback system, so that the shoots favored initially become even more favored in time."

Until the tree is tall.

"As the tree gets larger and larger," continues Steiner, "and this tip gets farther and farther away from the root system, its relative advantage becomes smaller and smaller, hydraulically, so its growth becomes less and less."

The taller the tree, the greater the difference needs to be between the water tension in the soil and that in the topmost leaf.

"At some point, the tree gets so tall it can't develop enough tension in its leaves to pull water effectively," explains Steiner. "The tip can no longer take advantage of its larger port. The number of hours in the day when leaves cannot photosynthesize, because they're too far away from the roots to get water, becomes more important.

"At that time, the tree stops growing up. It begins growing out in space. What was before a conical thing now becomes a globus.

"And the point at which this happens depends mostly on the water regime of the site. It's different for different species, but, basically, moist environments have tall forests and dry environments have short forests.

"You see, the tree has to grow, it has to extend, because you don't produce leaves on old wood. But old trees manage to do this without getting taller."

We drive into the forest to a small path through the trees. The day remains dark and chill, the undergrowth damp; the treetops rock in the wind. It was a good summer for rattlesnake sitings, Joyce remarks.

"I used to think a branch was a branch," Steiner muses, changing his oxfords for rubber-soled boots, "that if favored, any branch could become big. One of my earliest memories is transplanting a hickory tree into my parents' yard. I pruned the top out of it—I thought I needed to compensate for the loss of some of the roots. Well, it took about 10 years for that tree to start growing straight up again."

The path ends at a tower of orange scaffolding set on concrete footers between two black cherry trees. They're scrubby trees. Elsewhere in the Pennsylvania forests, a white pine I've read about stands 17 stories high—170 feet from root to crown. Not here. Tubes and wires funnel down from the leaf canopy, a mere 65 feet up, and from a 90-foot weather tower nearby, into a pre-fab gardener's shed. The shed rumbles busily. Joyce unlocks the door.

The wires feed strip charts and data recorders. Two shelves hold an instrument jungle: an old PC, photosynthesis monitors, devices to measure the water in a leaf. Joyce takes down a stem-flow gauge, a tube of white pipe insulation cut lengthwise and sealed with velcro strips; he splits it like a hot-dog bun to reveal heater strips and temperature recorders inside. Strapped around a branch, he explains, the tube heats the tree sap, taking the branch temperature above and below the hot spot. From the difference between the two, and a knowledge of the specific heat of water, Joyce can calculate the volume of water flowing through the branch.

"You can imagine the difference in the water transport systems of a 1-meter-tall seedling and a tall tree," he says.

He steps outside, points out the aluminum-foil-wrapped stem-flow gauges festooning the high branches. "First you have gravity to overcome. In addition, you have a longer pathway. Also, the structure is much more complex. There are microenvironments in the crown of a large tree: light and temperature differences because of shading from other leaves, differences in humidity. Then there's another biggy: Because large trees have large trunks and branches, with heartwood, they can store water."

Early in the morning, Joyce has found, water moves quickly out of small branches and twigs to evaporate into the air; to be ready for this morning surge, big trees continue sucking water into their thicker limbs at night.

"The large stems are being recharged," he explains.

"Smaller trees have very little storage, which means that the water lost needs to be replenished by immediate uptake. But large trees can lose water over the course of the day without immediately replenishing it."

He climbs quickly up the scaffold, snaking through a trap door to the first platform, about 25 feet up. Steiner and I follow. After a check on the equipment bolted to the scaffold, Steiner continues on up.

Vertigo claims me. The forest floor now seems far below, the scaffold shaky. Joyce insists he's had eight visitors on top at once, that he and the project's research associate, Todd Fredericksen, routinely tie themselves to the bucks and rappel off to reach the far branches. They've had two scares: Once they arrived to find the battery on the first platform had blown up in the night (the case lacked ventilation, hydrogen gas had built up inside); another day, up on top, overlooking the sea of leaves, they were mesmerized by the fantastic display of a far-off thunderstorm, letting it come a little close for comfort. Then, down below, there were the rattlesnakes. Twice they found them on the path.

Steiner halloos from the treetops. "Wow! Have you ever seen a cucumber magnolia with its top all loaded with fruits?"

I don't budge.

The scaffold and its attendant lab are funded by the EPA. Joyce, with money from the U.S. Forest Service, is "piggybacked" onto the larger study.

"But to learn how to scale, the hydraulic architecture question is a critical one."

Ozone enters a leaf through its open stomates. "The pattern of opening and closing over the course of the day," explains Joyce, "is related to the leaf's water tension. Leaves tend to close to keep water from evaporating. It's the frictional resistance in the water transport system that directly influences that open-closed pattern."

Thus the focus on the plumbing.

But it's not a strict pipe-to-pump relationship.

"One of the interesting things we're finding is that although the leaves on seedlings are taking up more ozone because they keep their stomates open more," Steiner says, "the seedlings aren't harmed more.

"What we find is that the leaves on a mature black cherry quit growing in June. The tree's total leaf complement is exposed to ozone over the whole season. But a seedling keeps growing through August. Its youngest leaves"—the ones budding out late in the summer—"get less ozone exposure. So the average amount of ozone exposure for all the leaves of a seedling is actually less than that for a mature tree.

"It sure is complicating our scaling study, this difference in the strategy of growth."

We drive to a second scaffold, from which saplings are monitored. It sits on the edge of a field of goldenrod and aster, planted spruces and pines, out of which a friend's setter one day flushed a "huge" woodcock, recalls Joyce. It flew right at him—"He had as much of an expression on his face as a woodcock can," he laughs—and swerved over his head. He searches the wet ground for the characteristic holes the bird makes when it probes the earth for worms.

We climb to the top of this lesser scaffold and lunch a comfortable 15 feet up.

"It was really neat," Joyce says, unwrapping a granola bar, "to see that the characteristic shape of a tree species is related to the construction of its internal water system—that you have this nested structure within the tree—that the pattern of water regulation in the branch is the same as the pattern in the whole tree."

Discussing the scaling study earlier he had said, "The branches may act like smaller trees rooted into the trunk, as if they were a population of seedlings."

"What's fascinating to me, in a tree," says Steiner, opening a small Snickers, "is that in this archetype of a random, wild form you have such a systematic internal pattern—not only systematic, but regulated.

"But the funny thing about living systems is there's no such thing as a cause or an effect. They're circular. They're completely circular. They're so integral you can't pull out one feature and say, 'This causes that.' Whenever I think I've found a cause, I also find it could be equally well considered an effect. Everything fits, and fits so well there's no beginning and no end."

So it is with water and wood. Perhaps the way the water flows through a tree does explain why an elm is trumpet shaped, a spruce is a cone. Or perhaps the trumpet and the cone explain the water's flow.

Steiner smiles. "Hydraulic architecture is only one part of that great circle." He finishes the candy.

"But it's neat to find something so nicely quantified and so systematic."

Kim Steiner, Ph.D., is professor of forest biology in the School of Forest Resources, College of Agricultural Sciences, 213 Ferguson Building, University Park, PA 16802; 814-865-9351. Durland Shumway, Ph.D., is a research assistant in the department of horticulture. Brian Joyce is a doctoral student in the intercollege graduate degree program in ecology. This research is funded by the U.S. Forest Service and the Environmental Protection Agency.